Semiconductor materials are used in numerous electronic applications including computers, sensors, and process control instrumentation. Increasing the speed of computers has necessitated the fabrication of increasingly smaller components coupled with the application of optical elements to more and more aspects of computer design. These technological prerequisites, as well as the need for better and more versatile sensors and process-control instrumentation, have brought with them requirements for semiconducting components with new or enhanced functionalities. The technological demands of high-speed computers have led to the development of techniques such as micromachining, vapor phase epitaxy, and nano-lithography, which allow the fabrication of increasingly smaller structures whereby bulk materials are engineered to provide submicron scale objects. Unfortunately, the technology required for molecule-discriminating devices such as sensors has not developed to the same advanced level. Most present-day molecule-discriminating devices contain one or more components composed of porous sintered aggregates of bulk semiconductors such as ZrO.sub.2 and SnO.sub.2. These materials have been described in the semiconducting art as "microporous", which describes the average dimensions of the random network of pores contained between the grains of the partially sintered component.
The term "microporous" has also been used in the semiconductor art to describe silicon metal that has been anodically etched in hydrofluoric acid so that only filamentous regions of nanometer dimensions remain. For example, Lehmann and Gosele describe the preparation of porous silicon. See Adv. Mater. 4(2), pp. 114-116 (1992). Evidence suggests that quantum confinement of electrons takes place in these silicon nanofilaments. Quantum confinement in semiconductors allows applications in LEDs, superlattice and quantum well lasers and other devices.
Applicants have taken a different approach to the design of new semiconducting functionalities which involves the use of crystalline materials that are nanoporous. By nanoporous is meant a material which has a crystallographically regular intracrystalline pore system whose pores have an average diameter of 2.5 to about 30 .ANG.. Zeolites are the best known nanoporous materials. Zeolites are materials which have a three-dimensional framework structure composed of aluminum, silicon, and oxygen and an intracrystalline pore system. The intracrystalline pore system may have pores in one, two or three crystallographic directions. The unique feature of these materials is that the pores in any one direction are crystallographically ordered. Other examples of crystalline nanoporous materials include aluminophosphates, silicoaluminophosphates, germanium and tin-based metal sulfides and selenides, and zinc phosphates. See U.S. Pat. Nos. 4,310,440; 4,440,871; 4,880,761 and 5,126,120, respectively.
Applicants have found that a subclass of these molecular sieves have semiconducting properties. In particular, they are characterized in that they have an optical band gap of greater than zero to about 5 eV. Because of their nanoporosity, these semiconducting materials can be used in molecule discriminating electronic and optoelectronic devices (as well as other electronic and optoelectronic devices), in which a conventional semiconductor is replaced by a nanoporous structure to allow a molecule discriminating electronic or optical response to the presence of physisorbed and chemisorbed molecular and ionic species.
The effects of physisorbed species on the electronic properties of a dense semiconductor is usually negligible. The attractive forces involved in physisorption are generally of the dipole and induced-dipole type and usually follow simple Lennard-Jones r.sup.-6 energy dependence. The adsorbing surface will undergo some slight polarization with minimum penetration into the adsorbent. Physisorption within a nanoporous semiconductor induces a polarization that will affect the electronic properties to a much greater magnitude than any bulk or microcrystalline semiconductor. A nanoporous semiconductor is in principal a three-dimensional surface that is only a few atoms thick between adsorption sites in adjacent void spaces. The volume percent of this type of semiconductor that is polarized is essentially proportional to the fraction of the atoms that are on the intracrystalline surface. The net effect of physisorption is a magnification of the polarization relative to dense semiconductors and a corresponding perturbation of the electronic properties.
Electronic perturbations will also occur in acid-base types of interactions or in electrostatic ion-dipole and ion-quadrupole interactions between adsorbates and nanoporous semiconductors. The semiconductor/adsorbate interactions outlined here would perturb the electronic properties of a nanoporous semiconductor in a measurable way.
Because of the weak or negligible electronic effects of physisorption in traditional dense, microcrystalline semiconductors, the materials have only been used in sensing devices where chemisorption or ionosorption takes place. The electrical properties of semiconductors can be profoundly affected by the presence of chemisorbed or ionosorbed species at the surface. The main reason for these effects is the limited number of charge carriers in intrinsic semiconductors and the resulting deep penetration of the space-charge layer when carders are trapped at the surface by chemisorbed electron acceptors or donors. The space-charge layer (the depth of which is called the Debye Length) arises from the transfer of charge to or from the chemisorbed molecule and the resulting potential radient that extends from the surface into the semiconductor. The Debye Length is usually characterized as the penetration depth corresponding to a potential barrier of 1/e times the surface potential value.
The Debye Length for semiconductors with band gaps of greater than 0.8 eV will be at least 5.6 .ANG.. Larger band gap materials, a category that includes many nanoporous chalcogenides, will display much larger Debye Lengths of the space charge layer as a result of chemisorption. The Debye Length of a space charge layer associated with appreciable chemisorption would penetrate the entire volume of a nanoporous semiconducting material.
Sintered polycrystalline compacts of dense semiconductors, such as SnO.sub.2, are widely used as combustible gas sensors. Polycrystalline compacts are used to maximize the amount of surface in contact with the atmosphere. The principal of operation is the conductance response to ionosorption of O.sub.2 (to form O.sub.2.sup.-) and the effect of reducible gases on the amount of adsorbed O.sub.2. The response of these semiconducting sensors is extremely dependent on the device fabrication technique and aging of the SnO.sub.2. Theoretical models for the function of these porous semiconducting sensors are based on random barrier networks and Schottky barrier conduction.
Random barrier networks are associated with the conductivity through variable radius intergrain "necks", formed during the sintering of the compacts. See, P. Romppainen and V. Lantto, J. Appl. Phys., 63, (1988), 5159-5165. The Schottky barrier model is characterized by surface potential barriers caused by O.sub.2 adsorption between the SnO.sub.2 grains. See, J. F. McAleer, P. T. Moseley, B. C. Tofield, and D. E. Williams, Bri. Ceram. Proc., 1985, 89-105. The electric conductivity is therefore a function of grain boundary characteristics in the Schottky model. Both of these models predict a drastic dependence of the device performance on the sintered microstructure of the semiconducting component.
Nanoporous semiconductors do not have a random network of barriers and their electrical response would be large enough to preclude the necessity for a large external surface area. The interconnections between secondary building units are crystallographically regular, rather than random, and are a function of the framework topology rather than the fabrication history. A sensing device could be based on a single crystal of a nanoporous semiconductor. Besides the advantage of miniaturization, nanoporous semiconductors can be used to detect or discriminate physisorbed species as well as chemisorbed species, because of the magnitude of the sorption-induced electronic effects.
The nanoporous materials which possess semiconducting properties include the chalcogenide (sulfur, selenium, or combinations thereof) materials described in U.S. Pat. No. 4,880,761 and the oxysulfide materials of U.S. Pat. No. 5,122,357. Other materials are polychalcogenides (sulfur and selenium) compounds of group VIII, IIB, IIIB, IVB and VB of the Periodic Table of the Elements, as well as metal oxides hollandite, psilomelane, and todorokite pore structures, or intergrowths of the structures.
The above-named materials can be prepared hydrothermally with the aid of a structure-directing cation, by solid state high temperature methods or in molten salt media. After crystallization, these nanoporous materials will contain at least some structure directing agent or cations in the intracrystalline pore system. Applicants have found that the band gap of these materials can be modified by changing the concentration of the structure-directing agent present in the nanoporous material.
The art contains reports of "framework" materials that have semiconducting properties. For example, S. Dhingra and M. G. Kanatzidis in Science, Vol. 258, pp. 1769-72 (1992) report that materials such as (Ph.sub.4 P)(M)(Se.sub.6).sub.2 have semiconducting properties. There are also reports of various metal oxides with the hollandite structure having semiconducting properties. These are: 1) Mat. Res. Bull., 25, 139-148 (1990); 2) Mat. Res. Bull., 18, 203-210 (1983); and 3) J. Mater. Chem., 2(10), 993-996 (1992).
Finally, U.S. Pat. No. 5,151,110 discloses using zeolites to adsorb chemical entities which gives rise to a mass change which change is detected by a piezoelectric substrate.